In my post about Always On GPS, I suggested sharing the location tables for WiFi between users. This would enhance the database and improve coverage and accuracy for this hybrid-location technique.
Besides sniffing for WiFi access points, an alternative is cellular cellsite detection, which, of course, is how mobile operator solutions work (in various forms).
Thank you Chris Peterson for pointing out Cell Spotting, which does exactly both: it detects cells and allows users to update a central database of cellsite information. Cell Spotting is an open project outside of operator control.
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